This invention relates generally to high-voltage electrical connections and to methods for fabricating such connections. More particularly, the present invention provides improved high-voltage electrical connections for high-voltage electrical devices, such as x-ray tube assemblies which are assembled using encapsulants, and methods for making such high-voltage connections.
High-voltage electrical devices, for example, devices operating at +/xe2x88x9210 kilovolts (kV) AC or DC or above, are common to many industries, for example, the power industry, the materials industry, and the analytical industry, among others. For example, x-ray producing devices, which are valuable tools used in a wide variety of industrial and medical applications, often operate with at least one component at high voltage, for example, at 50 kV or higher. X-ray producing devices are commonly used in areas such as diagnostic and therapeutic radiology, and materials testing, among others.
High-voltage devices may be fabricated with some form of encapsulation material, also known as an encapsulant, which provides a electrically insulating barrier around one or more components of a high-voltage device. Encapsulants may also minimize or exclude the presence of air about a high-voltage device, and thus minimize the potential for undesirable arcing and corona discharge to occur, may act as barriers to environmental contaminants, and may isolate components from shock or vibration loading. One encapsulant typically used in such devices is referred to in the art as a xe2x80x9cpotting materialxe2x80x9d. Potting materials are typically flexible, elastomeric materials, such as silicone-based material, that can be molded (that is, xe2x80x9cpottedxe2x80x9d) about one or more components of a high-voltage device. Though conventional potting materials provide electrical insulation, their chemistry is sometimes incompatible with the sheathing on electrical cables that are often used to connect the high-voltage device to, for example, a high-voltage power source.
Encapsulants may be curable encapsulants. Curable encapsulants may be provided in a liquid or semi-liquid form which is curable to a solid or semi-solid under certain curing conditions.
Typically, high-voltage devices require at least one electrical connection to a high-voltage power source, often by means of a specially-designed high-voltage cable. Typically, these high-voltage cables are constructed with layers of insulating materials, such as a sheathing made of silicone, polyethylene, or other insulating materials. Silicone-sheathed high-voltage cables are very flexible and their silicone sheathing is typically compatible with silicone potting materials, that is, a satisfactory bond can be established between the silicone sheathing and the silicone potting materials during fabrication. Direct bonding of the silicone sheathing of the cable to the potting material can provide a contiguous bonded connection which, most importantly, can minimize or prevent the formation of any undesirable air gaps between the sheathing and potting material.
In high-voltage electrical connections air gaps must be minimized or avoided entirely. The presence of any air gaps in high-voltage electrical connections introduces the potential for high-voltage electrical arcing or corona discharge at the interface of the cable and the high-voltage device. High-voltage arcing can reduce the power and voltage transferred to the high-voltage device, can damage the connection or high-voltage device, can provide an undesirable fire risk, or provide a safety risk due to electric shock. High-voltage corona discharge can attack insulation materials and cause the insulation materials to degrade. However, though high-voltage silicone-sheathed cables are less susceptible to the formation of air gaps, silicone-sheathed cables are very expensive. Furthermore, silicone-sheathed cables are difficult to adapt for use with a variety of standard power supplies, such as power supplies with air gap connectors. Other more cost-effective and readily adaptable high-voltage cables, such as polyethylene-sheathed cables, are readily available; however, the sheathing of such cable, for example, polyethylene sheathing, is extremely difficult to bond directly to most encapsulants, for example, extremely difficult to bond to an uncured encapsulant. Most notably, polyethylene sheathing does not bond to uncured silicone-type potting materials. Conventionally, the use of polyethylene-sheathed high-voltage cables with silicone-based potting materials typically is hindered due to the incompatibility of the two materials and the use of such materials is prone to the formation of undesirable air gaps and consequent arcing or corona discharge in the high-voltage connection.
Thus, there is a need in the art to provide a method and device for using a cost-effective and readily adaptable high-voltage cable, for example, a polyethylene-sheathed cable, to form an electrical connection between a encapsulated high-voltage device, for example, an silicone-encapsulated high-voltage device, and a high-voltage power source. There is also a need in the art for a method for bonding of high-voltage cable, for example, a high power polyethylene-sheathed cable, to a high-power encapsulated device, for example, a high power silicone-encapsulated device, that minimizes or prevents the formation of air gaps and provides a contiguous bonded connection.
The present invention provides methods and apparatus which address many of the limitations of prior art methods and apparatus.
One aspect of the invention is an electrical connector for an electrical device. In this aspect of the invention, the device includes a conducting element at least partially encased in a curable encapsulant and the electrical connector includes a cable having a conductor and a sheath surrounding the conductor, the sheath comprising a first material that is not bondable with the uncured encapsulant; and a sleeve mounted to the sheath, the sleeve comprising a second material that is bondable with the uncured encapsulant; wherein the sleeve and the encapsulant form an essentially air-tight connection between the sleeve and the encapsulant when the encapsulant is cured. In one aspect of the invention, the electrical device comprises a high-voltage electrical device, for example, an x-ray tube, an x-ray transformer, a high-voltage rectifier, high-voltage power supply, or microwave generator, among other devices. In one aspect of the invention, the sleeve is mounted to the sheath by means of an adhesive, for example, an epoxy resin adhesive. In another aspect of the invention, the encapsulant is silicone encapsulant. In one aspect of the invention, the first material is a dielectric material, for example, a polyethylene dielectric material. In one aspect of the invention, the second material comprises a silicone material, for example, a cured silicone encapsulant.
Another aspect of the invention is a sleeve for a cable for use in connecting the cable to a device, for example, a high voltage device, such as a an x-ray tube, an x-ray transformer, a high-voltage rectifier, high-voltage power supply, or microwave generator, among other devices. In this aspect of the invention, the device includes a conducting element at least partially encased in a curable encapsulant and the cable includes a conductor and a sheath surrounding the conductor. The sheath is made of a first material that is not bondable with the uncured encapsulant. The sleeve may be a cylindrical tube made of a second material which is bondable with the uncured encapsulant to provide an essentially air-tight connection between the sleeve and the encapsulant when the encapsulant is cured. In one aspect of the invention, the curable encapsulant comprises a silicone encapsulant. In one aspect of the invention, the second material comprises a cured encapsulant. In another aspect of the invention, the sleeve may include an end cap, for example, an endcap having a hole through which the cable conductor can pass.
Another aspect of the invention is a method for fabricating an electrical connection for a device, for example, a high-voltage device, such as an x-ray tube, an x-ray transformer, a high-voltage rectifier, high-voltage power supply, or microwave generator, among other devices. The device may include a conducting element at least partially encased in a curable encapsulant. The method of the present invention includes providing a cable having a conductor and a sheath surrounding the conductor, the sheath having an outside surface having an outside diameter and comprising a first material that is not bondable with the uncured encapsulant; providing a cylindrical sleeve having an inside diameter about equal to the outside diameter of the sheath and comprising a second material that is bondable with the uncured encapsulant; mounting the sleeve on the sheath to provide a sleeved cable; attaching the conductor of the sleeved cable to the conducting element of the device; and at least partially encasing the conducting element and the sleeved cable in an encapsulant wherein the sleeve and encapsulant bond to provide an essentially air tight connection between the sleeve and the encapsulant. The mounting step may include abrading the outside surface of the sheath or applying an adhesive to the outside surface of the sheath. The method may also include applying an adhesive to the outside surface of the sheath, for example, applying an epoxy resin to the outside surface of the sheath. In one aspect of the invention, attaching the conductor of the sleeved cable to the conducting element may be practiced by soldering, brazing, or welding the conductor to the conducting element. In another aspect of the invention, the conductor and conducting element may be attached by means of an adhesive, for example, an electrically-conductive adhesive, for instance Tiga Silver 901 electrically-conductive adhesive sold by Resin Technology of South Easton, Mass., or its equivalent. In one aspect of the invention, the step of at least partially encasing the conducting element and the sleeved cable in an encapsulant may be practiced by at least partially encasing the conducting element and the sleeved cable in an encapsulant or potting material, for example, a silicone potting material.
These and other embodiments and aspects of the present invention will become more apparent upon review of the attached drawings, description below, and